Rapid and precise quality control inspections of the soldering and assembly of electronic devices have become priority items in the electronics manufacturing industry. The reduced size of components and solder connections, the resulting increased density of components on circuit board assemblies and the advent of surface mount technology (SMT), which places solder connections underneath device packages where they are hidden from view, have made rapid and precise inspections of electronic devices and the electrical connections between devices very difficult to perform in a manufacturing environment.
Many existing inspection systems for electronic devices and connections make use of penetrating radiation to form images, which exhibit features representative of the internal structure of the devices and connections. These systems often utilize conventional radiographic techniques wherein the penetrating radiation comprises X-rays. Medical X-ray pictures of various parts of the human body, e.g., the chest, arms, legs, spine, etc., are perhaps the most familiar examples of conventional radiographic images. The images or pictures formed represent the X-ray shadow cast by an object being inspected when it is illuminated by a beam of X-rays. The X-ray shadow is recorded by an X-ray sensitive material such as film or other suitable means.
The appearance of the X-ray shadow or radiograph is determined not only by the internal structural characteristics of the object, but also by the direction from which the incident X-rays strike the object. Therefore, a complete interpretation and analysis of X-ray shadow images, whether performed visually by a person or numerically by a computer, often requires that certain assumptions be made regarding the characteristics of the object and its orientation with respect to the X-ray beam. For example, it is often necessary to make specific assumptions regarding the shape, internal structure, etc. of the object and the direction of the incident X-rays upon the object. Based on these assumptions, features of the X-ray image may be analyzed to determine the location, size, shape, etc., of the corresponding structural characteristic of the object, e.g., a defect in a solder connection, which produced the image feature. These assumptions often create ambiguities, which degrade the reliability of the interpretation of the images and the decisions based upon the analysis of the X-ray shadow images. One of the primary ambiguities resulting from the use of such assumptions in the analysis of conventional radiographs is that small variations of a structural characteristic within an object, such as the shape, density, and size of a defect within a solder connection, are often masked by the overshadowing mass of the solder connection itself as well as by solder connections on the opposite side of the circuit board, electronic devices, circuit boards, and other objects. Since the overshadowing mass is usually different for each solder joint, it is extremely cumbersome and often nearly impossible to make enough assumptions to precisely determine shapes, sizes, and locations of solder defects within individual solder joints.
In an attempt to compensate for such shortcomings, some systems incorporate the capability of viewing the object from a plurality of angles. The additional views enable these systems to partially resolve the ambiguities present in the X-ray shadow projection images. However, utilization of multiple viewing angles necessitates a complicated mechanical handling system, often requiring as many as five independent, non-orthogonal axes of motion. This degree of mechanical complication leads to increased expense, increased size and weight, longer inspection times, reduced throughput, impaired positioning precision due to the mechanical complications, and calibration and computer control complications due to the non-orthogonality of the axes of motion.
Many of the problems associated with the conventional radiography techniques discussed above may be alleviated by producing cross-sectional images of the object being inspected. Tomographic techniques such as laminography and computer-aided tomography (CT) have been used in medical applications to produce cross-sectional or body-section images. In medical applications, these techniques have met with widespread success, largely because relatively low resolution on the order of one or two millimeters (approximately 0.04 to 0.08 inches) is satisfactory and because speed and throughput requirements are not as severe as the corresponding industrial requirements.
In the case of electronics inspection, and more particularly, for inspection of electrical connections such as solder joints, image resolution on the order of several micrometers, for example, 20 micrometers (approximately 0.0008 inches) is preferred. Furthermore, an industrial solder-joint inspection system must generate multiple images per second in order to be of practical use on an industrial production line.
Laminography systems which are capable of achieving the speed and accuracy requirements necessary for electronics inspection are described in the following patents: U.S. Pat. No. 4,926,452, entitled, “AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS,” issued to Baker et al.; U.S. Pat. No. 5,097,492, entitled, “AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS,” issued to Baker et al.; U.S. Pat. No. 5,081,656, entitled, “AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS,” issued to Baker et al.; U.S. Pat. No. 5,291,535, entitled, “METHOD AND APPARATUS FOR DETECTING EXCESS/INSUFFICIENT SOLDER DEFECTS,” issued to Baker et al.; U.S. Pat. No. 5,621,811, entitled, “LEARNING METHOD AND APPARATUS FOR DETECTING AND CONTROLLING SOLDER DEFECTS,” issued to Roder et al.; U.S. Pat. No. 5,561,696, entitled, “METHOD & APPARATUS FOR INSPECTING ELECTRICAL CONNECTIONS,” issued to Adams et al.; U.S. Pat. No. 5,199,054, entitled, “METHOD AND APPARATUS FOR HIGH RESOLUTION INSPECTION OF ELECTRONIC ITEMS,” issued to Adams et al.; U.S. Pat. No. 5,259,012, entitled, “LAMINOGRAPHY SYSTEM AND METHOD WITH ELECTROMAGNETICALLY DIRECTED MULTIPATH RADIATION SOURCE,” issued to Baker et al.; U.S. Pat. No. 5,583,904, entitled, “CONTINUOUS LINEAR SCAN LAMINOGRAPHY SYSTEM AND METHOD,” issued to Adams; and U.S. Pat. No. 5,687,209, entitled, “AUTOMATIC WARP COMPENSATION FOR LAMINOGRAPHIC CIRCUIT BOARD INSPECTION,” issued to Adams. The entirety of each of the above referenced patents is hereby incorporated herein by reference.
In a laminography system, which views a fixed object and has an imaging area that is smaller than the object being inspected, it may be necessary to move the object around to position different regions of the object within the imaging area thus generating multiple laminographs, which when pieced together form an image of the entire object. This is frequently achieved by supporting the object on a mechanical handling system, such as an X-Y-Z positioning table. The table is then moved to bring the desired regions of the object into the imaging area. Movement in the X and Y directions locates the region to be examined, while movement in the Z direction moves the object up and down to select the plane within the object where the cross-sectional image is taken.
Several of the above-referenced patents disclose devices and methods for the generation of cross-sectional images of test objects at a fixed or selectable cross-sectional image focal plane. In these systems, an X-ray source system and an X-Ray detector system are separated in the “Z” axis direction by a fixed distance and the cross-sectional image focal plane is located at a predetermined specific position in the “Z” axis direction which is intermediate the positions of the X-ray source system and the X-ray detector system along the “Z” axis. The X-Ray detector system collects data from which a cross-sectional image of features in the test object, located at the cross-sectional image focal plane, can be formed. These systems postulate that the features desired to be imaged are located in the fixed or selectable cross-sectional image focal plane at the predetermined specific position along the “Z” axis. Thus, in these systems, it is essential that the positions of the cross-sectional image focal plane and the plane within the object, which is desired to be imaged, be configured to coincide at the same position along the “Z” axis. If this condition is not met, then the desired image of the selected feature within the test object will not be acquired. Instead, a cross-sectional image of a plane within the test object, which is either above or below the plane that includes the selected feature will be acquired. In some arrangements, less than optimal views of a solder joint may be imaged. Consequently, analysis of these less than optimal views can lead to inaccurate analysis of the associated solder-joint.
Presently, one technique commonly used for positioning the selected feature of the test object within the cross-sectional image focal plane, physically measures the “Z”-axis position of the selected feature. Using this measurement, the test object is then positioned along the “Z” axis such that the selected feature coincides with the “Z”-axis position of the cross-sectional image focal plane. Any of a variety of standard methods and instruments may be used to physically measure the “Z”-axis position of the selected feature of the test object. There are several types of commercially-available Z-ranging systems, which are used to determine the distance between a known location in “Z” and a feature on the surface, or just below the surface, of the test object. Such systems may be as simple as a mechanical fixing of the test object, a mechanical probe, a laser-based optical triangulation system, an optical-interferometric system, an ultrasonic system, among others. Any one of these “Z”-axis position measuring systems may be used to form a “Z-map” of the surface of the test object. The “Z-map” typically consists of an array of X and Y positions associated array of the Z-values of the surface of the test object. The locations (i.e., X, Y positions) are points on a plane shared with the test object that is substantially parallel to the cross-sectional image focal plane. The systems most commonly used in systems for cross-sectional image features on printed-circuit boards are laser-based triangulation range finders.
Range finders have been used in particular for cross-sectional X-ray image systems that are used to image electronic circuit board assemblies. Circuit-board assemblies are typically very thin in comparison to the surface area in which the components are mounted. Some circuit assemblies are made with very dimensionally-stable material, such as ceramic substrates. However, the majority of circuit-board assemblies are constructed with a material that is somewhat flexible or in some cases very flexible. This flexibility allows the board to develop a warp in the axis perpendicular to the major surface areas (i.e., the surface areas that contain interface pins) or the “Z” dimension. Additionally, some circuit board assemblies have variations in board thickness. In addition to electronic assemblies, there are many other objects that have dimensional variation on a scale that is significant when compared to the depth of field of the “Z” focal plane in cross-sectional X-ray imaging. By measuring the surface of a warped test object, the magnitude of the variation in the “Z” dimension can then be used to properly adjust the positional relationship of the test object with respect to the “Z” focal plane of the cross-sectional imaging system so that the desired image of the features of interest within the test object can be imaged.
Specifically, one such range finder system is designed for use in a system such as that described in U.S. Pat. No. 4,926,452 to Baker, et al. Hereafter referred to as the '452 patent. The '452 patent discloses a laminography system in which an X-ray based imaging system having a very shallow depth of field is used to examine solid objects such as printed-circuit boards. The shallow depth of field provides a means for examining the integrity of a solder joint without interference from the components above and below the solder joint. The material above and below the solder joint is out of focus, and hence, contributes to a more or less uniform background. To provide the needed selectivity, the depth of field of the laminographic-imaging system is on the order of less than approximately 2 millionths of an inch (2 mils.). Unfortunately, surface variations on the printed-circuit board often exceed this tolerance. To overcome this drawback, the surface of the printed-circuit board is mapped using a laser-range finder. The detailed laser-range finder generated map is then used to position the circuit board with respect to the X-ray imaging system such that the component of interest is in focus even when the board is translated from one field of interest to another.
One disadvantage of solder-joint inspection systems is the methodology used in determining whether a measured solder-joint feature is indicative of a solder joint that is “acceptable” or “defective.” Present solder-joint inspection systems apply a pre-set threshold to each solder joint feature in order to make these determinations. This methodology for identifying “defective” solder joints is problematic for determining when an individual solder joint on an array package is “defective.” This is especially evident in the accuracy rate of identified “open circuits” for solder joints associated with various array package types.
Ball-grid array (BGA) circuit packages, for example, have a planar-bottom face or mounting surface that is generally either square or rectangular in shape. This face may be covered with small spherical leads that carry electric signals to and from the integrated circuit that is a part of the “chip” or integrated circuit package. As is known, the planar bottom forms part of a substrate (typically a multi-layered substrate) to which an integrated circuit die is affixed.
A number of factors combine such that there are significant measurable differences from solder ball (i.e., a solder joint) to solder ball across the surface of an array package. First, the package material may warp resulting in a significant variation in the relative distance between the mounting surfaces of the package and a printed-circuit board. Generally, BGA packages warp such that the edges tend to pull away from the mounting surface of the printed-circuit board (i.e., the edges turn upward). However, it should be appreciated that BGA-package warp can occur in a host of various ways with the distance between the mounting surfaces of a BGA package and a printed-circuit board varying in a number of different ways across the mounting surfaces. Other packages, such as, but not limited to column grid array (CGA), flip-chip, chip scale packages (CSPs), and quad flat packs, etc. may also suffer from warp.
In addition to package warp, the printed-circuit board material may warp. It should be appreciated that these two conditions are not mutually exclusive of each other. Stated in another way, both the printed-circuit board (i.e., a mounting surface) and the package may be warped. Furthermore, either of the printed-circuit board and/or the package may suffer from “tilt.” A “tilt” condition is present when the mounting surface elevation (i.e., height in the “Z” dimension) varies from one edge to an opposing edge on either a package or a printed-circuit board.
These and other factors may work together such that a printed-circuit assembly may contain a plurality of multi-pin devices (e.g., array packages) with a significant variation in one or more solder-joint measurements. For example, in the case of substantially uniform (in volume) solder balls applied to the mounting surface conductors of a BGA package, an increase in the distance between the mounting surfaces of the BGA package and the printed-circuit board may cause one or more solder balls to stretch such that its diameter is less than that of the solder ball in its pre-reflow condition. Generally, the solder balls collapse from their pre-reflow condition. The balls closer to the center of the array typically collapse more than the solder balls closer to the external edges of the array.
Despite a wide range of measured solder-joint characteristics across the multitude of connecting pin locations on an array package, the associated conductors on the printed-circuit assembly and the array package may be adequately connected, both structurally and electrically, via the corresponding solder-joint. Any associated differences in the various measurements of the solder-joints (in those situations where corresponding conductors are adequately connected) is representative of an acceptable variation across the plurality of solder-joint measurements.
The wide range of acceptable solder-joint measurements makes it extremely difficult to set pass/fail thresholds for various measurements of solder-joints that accurately reflect the actual electrical and physical condition of each individual solder-joint. In practice, the accurate detection of “open circuit” conditions with solder-joint inspection systems for BGA devices has been problematic. Not only is it time consuming to rework each BGA package identified as having one or more “defective” solder-joint test results, short of a full functional test of the printed-circuit assembly (i.e., a completely populated printed-circuit board), or in-circuit test of the part, which requires 100% electrical test access, no non-destructive verification method exists to independently confirm the accuracy of the defect conclusion.
In response, some manufacturers have used oval shaped pads with various printed-circuit devices. The oval shaped pads cause solder balls to form oval shaped solder joints when the pad is properly bonded by the solder. The oval shaped pads permit detection of “open” solder joints as the associated solder image remains circular. Despite this and other improvements in detecting defective solder joints, it would be desirable to have an improved system and method for improving the accuracy of solder-joint inspection systems in accurately identifying defective solder-joints used to physically and electrically connect various printed-circuit devices on printed-circuit boards that accounts for acceptable variation in measured solder joints on a printed-circuit assembly.